The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent that it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Exhaust components, such as exhaust manifolds, turbochargers, and catalytic converters are provided downstream from engines to direct and guide exhaust gas flow for further treatment or use and are subject to high temperature. Exhaust manifolds are commonly made from cast iron for high volume production engines. Among the commonly used cast iron material for the exhaust manifolds is silicon-molybdenum cast iron (“SiMo cast iron”). SiMo cast iron becomes weaker as the temperature increases. As a result, the SiMo cast iron is subject to damage from oxidation, decarburization, and coarsening. The duration of time at high temperature determines the amount of material damage that accumulates. The accumulation of damage and the elevated temperature strength (the thermal strength) of the material are important factors in evaluating durability of the exhaust component.
As automotive companies increase the gas temperatures of their engines to improve efficiency and reduce exhaust emissions, more manifold applications are exceeding the practical working (temperature) limit of cast iron. The temperature distribution in the manifolds is not uniform and some peak temperature areas receive more heat than other areas in the manifolds. SiMo (silicon-molybdenum) cast iron exhaust manifolds have an AC1 temperature of approximately 830-840° C. The AC1 temperature is the temperature at which the ferritic microstructure starts to be converted into austenite. Since a typical maximum gas temperature of the manifold outlet for a current North American gasoline engine is about 900° C., it can be shown that most areas of the manifold will be below the AC1 temperature.
Currently, if a material such as cast iron is inadequate for the peak temperature, the entire manifold has to be made from a higher grade material (e.g., Ni-Resist, cast steel, or fabricated stainless steel). Therefore, the manufacturing costs for exhaust manifolds for high temperature applications are significantly increased.
Single material cast exhaust components can suffer severe damage in regions of local high temperature and large thermal gradients such as the outlet or along the bifurcation. The high temperature promotes oxidation and the thermal gradients introduce local strains that may make the oxide layer less adherent. If spalling of the oxide occurs, particles are released into the exhaust gas stream that may bombard and damage downstream components such as turbochargers and catalytic converters.
The oxidation, particle coarsening, and decarburization that occurs locally in the high temperature regions can significantly degrade the local material properties over time. This may result in premature cracking and warpage, both of which can reduce component durability performance. These effects, in turn, may result in exhaust gases leaking to the environment (through a crack or loss of sealing) or allow exhaust gas to communicate (travel) between separated runners or chambers (either will negatively influence system performance). If large thermally induced strains are co-located with the manifold areas with degraded material properties, component failure by cracking is common.